Electromagnetic waves are attenuated, i.e. their energy is reduced, when they pass through a grounded conductor.
The most straight forward form of EMI/RFI shielding is a metal enclosure. A good example of this is shielded twisted pair wiring, where a twisted –pair wire is put inside a metal tube. The EMI is first reduced through the twisted pair technique, and then the energy of the remaining EMI generated by the wires is then eliminated almost completely as it passes through the metal enclosure.
Most all electronic devices to require some level of shielding for the overall device, however, they also commonly require shielding at the circuit level. When EMC has not been optimized by proper circuit design, individual components and areas of a circuit must be shielded from each other.
Here is a good example of board level shielding of individual components:
Just as twisted pair wire is commonly shielded to deal with residual EMI, electronic devices with board level shielding, are still assembled in shielding enclosures to deal with residual EMI, a necessary step in FCC or EC compliance.
As with twisted pair wiring, the straight forward method of doing this is to enclose the finished device in a metal box or enclosure. Such enclosures generally have to be specifically designed for the device in question. Many companies specialize in this area of engineering.